Conjugated diene, such as 1,3-butadiene, isoprene and chloroprene as unsaturated hydrocarbons, are liable to accidentally generate porous insoluble polymer, known as popcorn polymer, both in liquid and gas phases. This is particularly the case in industrial distillation that satisfies various conditions liable to generate the popcorn polymer, such as suitable temperature at operation, high purity monomer, and coexistence of gas and liquid phases, moisture ingredient, and existence of iron rust.
Popcorn polymerization can result from the action of a variety of factors on the concerned monomer, for example oxygen, heat and rust as well as popcorn polymer particles already present in the monomer, which catalyze popcorn polymer formation. Specifically, it appears that the presence of one or more initiators e.g. water, oxygen, or hydrogen peroxide, results in the formation of popcorn polymer “seeds” in the organic material. The seeds themselves then perpetuate polymerization, without further requiring an initiator and/or a cross linking agent; they serve as sites for further polymerization. As the particular mechanism, it is believed that monomer diffuses through the surface of the growing polymer mass, and is added to the polymer at the centre thereof. For this reason, such polymerization is referred to as occurring “from the inside out”. Consequently, there is continued incorporation of monomer into the polymer phase, leading to build-up of the popcorn polymer. This continuous incorporation of monomer, added with the cross linking, implies high internal mechanical strains. These strains explain why the polymers breaks, producing new popcorn polymer seeds. The result is a hard polymeric foulant, which can cause serious equipment and safety concerns if left unchecked.
A particular problem attendant upon popcorn polymer formation is its extreme resistance to deactivation, once present in a system. Some of the seeds become attached to the processing and handling equipment, and cannot be readily removed by mechanical means; moreover, being insoluble in most common solvents, they are virtually impossible to wash out by use of such solvents. Even after equipment and storage facilities have been cleaned thoroughly, residual particles of popcorn polymer remain, and promote unwanted polymer growth. Trace particles remaining in the equipment will stay active for long periods without the presence of monomer, and serve to initiate polymerization when once again contacted therewith.
Popcorn polymer formation is especially critical in the case of conjugated diene monomers, such as 1,3-butadiene or isoprene. Here, popcorn polymerization may be responsible for pipe work and reactors becoming plugged and for tank charges polymerizing wholesale and the tanks concerned bursting as a consequence.